Polyvinyl alcohol (PVOH) is generally prepared by the alcoholysis or saponification of a polyvinyl ester, usually polyvinyl acetate. The conversion of the polyvinyl ester to polyvinyl alcohol is typically effected in methanol in the presence of an alcoholysis catalyst such as sodium hydroxide or sodium methylate. The products of this reaction are the desired polyvinyl alcohol and methyl acetate.
While the polyvinyl acetate is completely soluble in the methanol, the resulting polyvinyl alcohol is less soluble in the mixture of unreacted methanol and by-product methyl acetate. The fully alcoholyzed, or hydrolyzed, (93 to 100%) polyvinyl alcohol is often obtained from the reaction in the form of a slab of rubbery gel which still contains a large amount of solvent. This polyvinyl alcohol slab is cut up into small particles and placed in a slurry tank along with a liquid medium which is 60 to 65% methanol and 35 to 40% methyl acetate. The resulting slurry is held at about 110.degree. to 120.degree. F. (43.degree.-49.degree. C.) for a period of 15 to 30 minutes in order to remove residual catalyst and sodium acetate.
The polyvinyl alcohol solids are then separated from the slurry mother liquor in a centrifuge followed by drying in a heated dryer until a solids content of about 98% is attained.
The use of dryers by the industry to reduce the solvent level in the virgin polyvinyl alcohol has several disadvantages. It is virtually impossible to increase the production rate without increasing the dryer temperature or enlarging the dryer. Increased dryer temperature is detrimental since it tends to affect adversely polyvinyl alcohol properties such as solubility, color and burnt particle content.
Burnt particles of polyvinyl alcohol result from the particles sticking to the heated walls of the dryer and decomposing. This sticking characteristic of the polyvinyl alcohol particles is dependent upon the temperature of the dryer walls and the polyvinyl alcohol composition. The temperature at which the particles stick to the walls is termed the sticking temperature. Currently, the only method to avoid burnt particles is to lower the drying temperature. But this action lowers production rate. The problem is to increase the sticking temperature of the polyvinyl alcohol particles thereby reducing the burnt particle count without lowering the drying temperature or reducing production rate.
Another problem is to raise the sticking temperature of the polyvinyl alcohol without decreasing its cold water solubility and/or its low temperature water solubility. The ideal solution to the problem would be to increase the sticking temperature while increasing the cold water solubility and the low temperature water solubility. Cold water solubility is the solubility of dry polyvinyl alcohol in water @25.degree. C. Low temperature water solubility is the solubility of dry polyvinyl alcohol in water @60.degree. C. Increasing the cold water solubility and the low temperature water solubility is important because it allows polyvinyl alcohol users to obtain full solubility at temperatures lower than 90.degree.-95.degree. C. thus affording a savings in energy.
U.S. Pat. No. 3,156,678 discloses the alcoholysis of polyvinyl acetate in a solvent system containing methanol and methyl acetate.
U.S. Pat. No. 3,497,487 discloses that the water slurrying properties of particulate polyvinyl alcohols which are at least 70% alcoholyzed are improved by heating a slurry of the polyvinyl alcohol in a liquid medium which is a 2 to 20% solution of acetic acid in a solvent comprising methanol, methyl acetate, or a mixture of methanol and methyl acetate, then separating the polyvinyl alcohol from the liquid medium.
U.S. Pat. No. 3,654,247 discloses the cold water solubles content and/or the cold water imbibing tendency of a 93 to 100% alcoholyzed polyvinyl alcohol are reduced by heating at a temperature of 70.degree. to 190.degree. C. a dispersion of the polyvinyl alcohol in a liquid solvent comprising 30 to 100% methanol, 0 to 13% water and 0 to 45% of a C.sub.2 -C.sub.5 monohydric alcohol, methyl acetate, acetone, ethylene glycol, dimethyl ether, toluene, methylene chloride, or the like. The examples show that the polyvinyl alcohols prepared by the alcoholysis of polyvinyl acetate were vacuum dried at about 80.degree. C. prior to the heat treatment of the polyvinyl alcohol dispersion.
Canadian Pat. No. 795,385 discloses polyvinyl alcohol having a low swelling degree, little discoloration and no lumps is prepared by adding 5 to 15 wt % water to polyvinyl alcohol after saponification and before drying the composite by heating. The water could also be added in admixture with methanol and/or methyl acetate to avoid lumping.